This invention relates to video surveillance systems. It relates more particularly to the recording of information from a plurality of television cameras in sequence so that occurrences at different locations can all be monitored quasi-continuously.
Many stores and banks position television cameras at several critical locations such as tellers' cages, vault areas, check cashing windows, etc. Pictures from all of these locations are displayed on television monitors or are recorded on video tape for later display in the event that they are to be subsequently reviewed. Thus, if there is a holdup or a bad check is passed, the recorded pictures may help identify the culprits.
A preferred system applies the video signals from a plurality of television cameras in sequence to the video tape recorder and associated television monitor. Normally, the cameras run continuously and the system operates unattended for many hours before tape replacement is required. A sequential video recording system such as this is sold by QSI Systems, Inc., West Newton, Massachusetts under its model designation 800. It is described in detail in copending application Ser. No. 535,989, mailed Dec. 17, 1974, entitled SEQUENTIAL VIDEO SWITCHING SYSTEM, which application is owned by the assignee of the present application.
When recording television pictures during a surveillance operation, it is desirable to employ a time-lapse recorder so that the system can operate for a long period without changing the video tape. However, when a time lapse recorder is used in conjunction with a sequential video recording system such as the one described in the above pending application, there is a drastic drop in the efficiency of the system and in the amount of useful information recorded on the video tape.
This drop in efficiency occurs because the system switches from one camera to another in the switching sequence at a random rate which the operator selects, depending upon the particular circumstances. However, at the same time, the time lapse recorder is recording frames from the television cameras at a gating rate that will minimize tape usage. If the sequential switching rate beween cameras is less than the gating rate of the time lapse recorder, information from the same television camera may be recorded twice, three, or even more times in a row during successive recorder gating intervals, so that the outputs of the other television cameras in the sequence which could contain critical information may not be recorded for a relatively long period of time.
On the other hand, if the sequential switching rate is faster than the gating rate of the time lapse recorder, the output of a particular camera in the sequence may not be recorded for a number of cycles in the switching sequence, again resulting in the loss of potentially important recorded information.
Also, when using a sequentially switched recording system, some small amount of information is lost during the actual switching operations. During an emergency situation, it would be highly desirable if even this small amount of data could be recorded so that there would be virtually no gaps in the information on the tape.